This invention relates generally to lithographic printing apparatus and, more particularly, to a novel fluid dampener system for delivering an even, uniform film of dampener fluid onto a printing cylinder and the printing plate mounted thereon to eliminate the problem of edge scumming on the printed web.
Conventional lithographic printing apparatus normally includes a rotating plate cylinder on which the printing plate is mounted, an ink delivering system for feeding ink to selected areas on the printing plate, and a fluid dampener system for delivering fluid such as water to other selected areas of the printing plate on which no ink is to be deposited. A common form of fluid dampener system is known as a brush dampener unit and consists of a slowly rotating pan roller which is partially immersed in a pan of dampener fluid. As this roller slowly rotates, it carries on its outer circumferential surface a film of dampener fluid upwards from the pan. A rapidly rotating brush roller consisting of helically mounted bristles is rotated in contact with the pan roller, the bristles of the rotating brush roller wiping the dampener film off the pan roller and flinging it in the form of droplets or a mist onto another rotating drum. The dampener film is transferred from the drum to an engaging rubber-covered form roller and thence to the printing plate mounted on the plate cylinder.
This type of fluid dampener system is known to be confronted with the problem of edge scumming, that is, the unwanted presence of ink on the edges of the printed web which occurs because of the uneven delivery of dampener film across the width of the form roller to the printing plate.
When printing on a full-width web at or close to the design limits of the printing machine, the amount of dampener fluid delivered at the ends of the print cylinder is insufficient to prevent the buildup of ink at the edges. There may be several reasons for this occurrence, but one reason is felt to be the higher temperatures at the ends of the machine elements, including the pan roller, the brush roller, the drum, the form roller, and the print cylinder, which higher temperatures cause greater evaporation of the dampener fluid in the end areas of the rollers.
Prior attempts to overcome this problem have been unsuccessful. For example, increasing the speed of the pan roller to a point where the dampener fluid feed rate allows the edges on the web to clean up has produced other problems such as overfeed of fluid in the central area of the print cylinder which results in a deterioration in print quality in that area. Another proposal to add additional bristles at the ends of the brush rollers to try to feed more dampener fluid in those areas has been ineffective, because the speed differential between the brush roller and the pan roller is so great that the normal brush is already taking away all the dampener fluid delivered to it.